1. Field of the Invention
The present invention relates to electrical contact elements for electrical devices, and more particularly to lithographic-scale, microelectronic spring contacts with improved contours.
2. Description of Related Art
Recent technological advances, such as described in U.S. Pat. No. 5,917,707 to Khandros et al., have provided small, flexible and resilient microelectronic spring contacts for mounting directly to substrates, such as semiconductor chips. The '707 patent discloses microelectronic spring contacts that are made using a wire bonding process that involves bonding a very fine wire to a substrate, and subsequent electroplating of the wire to form a resilient element. These microelectronic contacts have provided substantial advantages in applications such as back-end wafer processing, and particularly for use as contact structures for probe cards, where they have replaced fine tungsten wires. It is further recognized, as described, for example, in U.S. Pat. Nos. 6,032,446 and 5,983,493 to Eldridge et al, that such substrate-mounted, microelectronic spring contacts can offer substantial advantages for making electrical connections between semiconductor devices in general, and in particular, for the purpose of performing wafer-level test and burn-in processes. Indeed, fine-pitch spring contacts offer potential advantages for any application where arrays of reliable electronic connectors are required, including for making both temporary and permanent electrical connections in almost every type of electronic device.
In practice, however, the cost of fabricating fine-pitch spring contacts has limited their range of applicability to less cost-sensitive applications. Much of the fabrication cost is associated with manufacturing equipment and process time. Contacts as described in the aforementioned patents are fabricated in a serial process (i.e., one at a time) that cannot be readily converted into a parallel, many-at-a-time process. Thus, new types of contact structures, referred to herein as lithographic-scale microelectronic spring (or contact, or spring contact) structures, have been developed, using lithographic manufacturing processes that are well suited for producing multiple spring structures in parallel, thereby greatly reducing the cost associated with each contact. Exemplary lithographic-scale spring contacts, and processes for making them, are described in the commonly owned, co-pending U.S. patent applications “LITHOGRAPHICALLY DEFINED MICROELECTRONIC CONTACT STRUCTURES, Ser. No. 09/032,473 filed Feb. 26, 1998 by Pedersen and Khandros, and “MICROELECTRONIC CONTACT STRUCTURES”, Ser. No. 60/073,679, filed Feb. 4, 1998 by Pedersen and Khandros, both of which are incorporated herein, in their entirety, by reference.
In general, lithographic processes allow for a great deal of versatility in design of spring contacts, which in turn permits numerous improvements over prior art designs. For example, although prior art lithographically formed structures in general typically have essentially flat rectangular cross-sections, contoured non-rectangular cross-sections are desirable for many spring contact applications. For a given thickness of resilient material, a lithographic type spring contact can be made stiffer and stronger by providing it with a suitably contoured, non-rectangular cross section. Other performance benefits may be realized by utilizing various other more complex shapes. However, prior art manufacturing methods are unsuitable for making lithographic type spring contacts with such suitably contoured, non-rectangular cross-sections, and other types of more complex shapes. Additionally, prior methods, for example, as disclosed in the above-referenced U.S. patent applications 09/032,473 and 60/073,679, fabricate the spring structures using a series of lithographic steps, thereby building up a z-component extension (i.e., extension of the spring tip away from the substrate surface) with several lithographic layers. However, the use of multiple layers adds undesirable cost and complexity to the manufacturing process. Layered structures are also subject to undesirable stress concentration and stress corrosion cracking, because of the discontinuities (i.e., stepped structures) that result from layering processes.
A need therefore exists for method of making microelectronic spring structures more quickly and easily by eliminating process layering steps and the associated costs, while providing springs with improved properties, such as improved strength, stiffness, resistance to stress concentration cracking, and elastic range. Additionally, a need exists for a method of making lithographically formed, microelectronic spring structures with defined contoured surfaces and more complex shapes.